My invention relates to the art of hot stamping, also known as hot-press printing, and is more specifically directed to a method of, and apparatus for, repeatedly imprinting characters on a continuous strip of plastics, paper or other material by pressing heated type against a printing ribbon placed in superposition on the strip.
A well known form of the printing ribbon used with hot stamping apparatus of the kind under consideration comprises the laminated layers of plastics or like backup material, parting or releasing material, metal, and adhesive. As heated type is pressed against the ribbon upon a desired surface, the metal layer is transferred and attached to the surface via the adhesive in the exact shape of the relief character or characters on the typeface. In another known form of printing ribbon, the aforesaid layers of metal and adhesive are replaced by a single layer of mixed pigment and adhesive.
For repeatedly imprinting characters on a continuous strip of plastics or like material by use of such printing ribbon, there has been suggested a rotary hot-stamping machine wherein the strip is fed at constant speed over a cylindrical platen in the same direction as the direction of travel of the printing ribbon from payoff reel to takeup reel (Japanese Patent Publication No. 49-14891). Type is mounted on a rotary type carrier over the platen so as to be pressed against the printing ribbon placed in superposition on the strip passing over the platen.
The operation of this prior art hot stamping machine is such that, while being pressed by the heated type, the ribbon is thereby transported a predetermined distance, which is considerably in excess of the width of the character or characters on the typeface, with the strip toward the takeup reel. This takeup reel is then actuated to wind up the ribbon by a length corresponding to the width of the character on the typeface. Further, when released by the type, the ribbon is fed back toward the payoff reel by reciprocating pullback means, only to such an extent that the type will be subsequently pressed against the ribbon at a region immediately adjoining its previously used region. The pullback means further operates to draw the ribbon from the payoff reel by a length corresponding to the width of the character.
The hot stamping machine according to the mentioned Japanese patent has the advantages of high speed operation and wasteless use of the printing ribbon, but is not free from some shortcomings. For proper operation of the machine, a braking force must be applied to the payoff reel to arest its rotation while the ribbon is being pulled back toward same. This braking force must be greater than the force of the pullback means required to detach the ribbon from the strip of desired material and to pull back the ribbon the required distance toward the payoff reel. If the braking force were too great, however, the pullback means would be unable to draw the ribbon from the payoff reel. The magnitude of the braking force on the payoff reel is therefore in need of fine adjustment.
An even more serious problem arises as the printing operation proceeds, with the gradual decrease in the radius of the roll of the printing ribbon on the paypoff reel, because then a correspondingly increasing force is required for unwinding the ribbon. Since the force of the pullback means is usually constant, the magnitude of the braking force on the payoff reel must be decreased in step with the decrease in the radius of the ribbon roll thereon. If the braking force is not properly regulated as above and becomes too great or too small in relation to the force of the pullback means, the machine will become inoperable.